Approximately eighty percent of the refracting power of the eye is at the cornea. When the cornea is misshapen or the axial length of the eye is too long or short, or the lens of the eye is functioning abnormally, the refractive errors of myopia, astigmatism or hyperopia can result. Spectacles correct refractive errors by refracting the light with a lens before it reaches the cornea in order to change the angle at which light enters the cornea. Contact lenses correct refractive errors of the eye by replacing the misshapen cornea with the front curve of a contact lens which is calculated to render the eye emmetropic (a state where no visual correction is necessary). When the lens is taken off, however, the cornea is still misshapen or defective and refractive errors still exist.
The cornea is very pliable and can be reshaped with a series of progressive contact lens changes. This procedure is known as Orthokeratology. The methods of Orthokeratology without the use of enzymes or other agents originated in 1962 as an extension of normal contact lens use. Orthokeratology is generally defined as the therapeutic use of contact lenses to reshape the corneal curvature, thereby improving refractive errors of the eye. Dr. Charles May and Dr. Stuart Grant are credited with pioneering the process. University and clinical level research over the next 20 years confirmed the safety, effectiveness, and retention of this procedure. Orthokeratology has today become a contact lens specialty practice for a limited number of private practitioners primarily in the United States.
The traditional Orthokeratology procedures use a series of progressive contact lens changes to reshape the cornea, producing less curvature and a more spherical shape. This reduces or eliminates myopia and astigmatism and improves natural vision. Retainer contact lenses are then worn to stabilize the results. The contact lenses are rigid gas permeable material with no enzyme/agents. The program length varies from six to eighteen months with progressive contact lens changes and examinations each two to six weeks.
A common fitting formula for Orthokeratology is as follows:
Lens Base Curve In Diopters=Flattest central corneal curvature in diopters to 1.0 diopter flatter.
Lens Diameter=Base curve in mm+1.5 mm ##EQU1##
Thickness=.18 mm for 0 power-subtract .01 mm for each 1 diopter minus; add .02 mm for each 1 diopter of plus
Intermediate Curve=Base Curve in mm+1.5 mm, width=.35-.5 mm
Peripheral Curve=Base Curve in mm+3.0 mm, width=.35-.5 mm
As the Orthokeratology program progresses, for myopia new contact lenses are refit with flatter curvatures, less correction, larger diameters, and greater thickness. The patient's central corneal curvature continues to lessen (flatten and become more spherical), myopia and astigmia are reduced, and unaided (natural) visual acuity improves significantly. When.maximum desired results are achieved or the patient ceases to improve, retainer contact lenses are worn full time or part time to stabilize results.
For traditional Orthokeratology procedures, university and clinical research indicates the following limits of change: 4 diopters of myopia and 2.5 diopters of astigmatism change, no appreciable hyperopia change, 2 diopters of central corneal change, and 9 lines of unaided visual acuity change on the Snellen chart. Regression may occur in hours or days if retainer lenses are not worn.
Notwithstanding the foregoing, there remains a need for an improved method of correcting refractive errors in the eye nonsurgically which can correct larger amounts of refractive errors, produce relatively permanent results, in a much shorter period of time.